The present invention pertains to a coated cemented carbide metal cutting tool for the machining of steels in general when a high wear resistance as well as a large toughness behaviour of the cutting edge are required. The tool is particularly suitable for turning of stainless steels.
When cemented carbide cutting tools are used in the machining of steels, the tool is worn by different mechanisms such as abrasive and chemical wear, chipping and fracturing of the cutting edge. For a coated tool normally having thin surface layers of wear resistant carbide, nitride, carbonitride and/or oxide compounds formed by various vapor deposition techniques, the coating contributes to increase the abrasive wear resistance. But it also acts as a thermal barrier for the diffusion of heat from the cutting surface into the underlying cemented carbide substrate. A high temperature within the edge region in combination with high cutting forces result in an increase of the creep deformation within the affected surface region of the substrate and the cutting edge deforms plastically.
The cutting of stainless steel is considered to be a particularly difficult machining operation since in addition to the above mentioned wear mechanisms, adhesive wear and plastic deformation are also prominent factors. Adhesive wear occurs when smearing materials like stainless steels during the cutting operation continuously adhere to and tear off material from the cutting edge. A short tool life is therefore very frequent when machining stainless steels. Furthermore, austenitic and so called duplex stainless steels exhibit strong deformation hardening which results in high contact forces, e.g., between the chip and the surface of the tool. When cutting such workpiece materials at high cutting speeds, considerable thermal energy is transferred to the cutting edge and, in combination with the high cutting forces, the tool edge may partly or entirely deform plastically. Deformation of the edge, which is mainly controlled by the properties of the surface region of the substrate, results in larger cutting forces and thus a reduced tool life. A large requirement of plastic deformation resistance is in clear contrast to a large requirement of edge toughness.
Edge toughness is also required in order to withstand mechanically induced damage of the edge outside the direct area of contact between the tool and the workpiece. This will reduce the number of edges to be used on the insert and, accordingly, the productivity of the tool. Damages of this kind, normally denoted chip hammering and chip jamming, may partly or wholly be avoided by selecting a proper microgeometry of the active edge surfaces of the insert but in several cases this possibility is not enough. U.S. Pat. No. 5,786,069 describes a coated turning insert suitable for turning of forged components of stainless steel. The insert has a cemented carbide substrate containing 2-10 wt-% cubic carbides of groups IVb, Vb and/or VIb of the periodic table (γ-phase), 5-11 wt-% cobalt binder and balance tungsten carbide (WC). The substrate has a highly tungsten alloyed binder phase and in one embodiment the as-sintered microstructure exhibits a 15-35 μm deep Co enriched surface zone free of γ-phase. The coating consists of an inner layer of Ti(C,N,O) with columnar grains and a top layer of finely grained κ-Al2O3. However, regarding the cemented carbide substrate, the combination of a high nominal content of binder phase and a thick γ-phase depleted surface zone followed by a zone characterized by a high concentration peak of γ-phase, all in relative terms, does not favor the resistance against plastic deformation. This will result in rapid wear and short tool life for machining austenitic and duplex stainless steels at high cutting speeds.
Multilayer coatings comprising first and second layers of different materials which are alternately laminated on the substrate, each of the first layers having a first thickness and each of the second layers having a second thickness are known. The two layers should preferably have a different crystal structure and/or at least different lattice spacings. One example is when the Al2O3 growth periodically is interrupted by a short TiN deposition process resulting in an (Al2O3+TiN)n multilayer structure see, e.g., Proceedings of the 12:th European CVD Conference page pr. 8-349. GB 2048960A discloses a multilayer coating with a multiplicity of alternating layers of 0.02 to 0.1 μm consisting of hard material of different compositions. In U.S. Pat. No. 4,984,940, Bryant et al. disclose a cutting insert composed of a cemented carbide substrate with 6.1-6.5 wt % cobalt, a coating including a base layer of titanium carbonitride followed by a multilayered coating. Said coating consists of a plurality of alumina layers separated from and bonded to each other by a group IVb metal nitride, such as titanium nitride.
A cemented carbide substrate with a coating comprising 6-8 alumina layers is also claimed in U.S. Pat. No. 5,700,569. EP-A-1103635 describes a cutting tool consisting of a cemented carbide substrate with 9.0-10.9 wt % cobalt and a coating comprising a medium temperature CVD (MTCVD) deposited TiCN-layer and a multilayer composed of totally 7-41 layers of α-Al2O3 and TiN or Ti(C,N).
Smoothing of coatings by mechanical post treatment in order to e g minimize the friction between the tool and the workpiece is disclosed in EP-A-127416, EP-A-298729, EP-A-693574 and EP-A-683244.
U.S.-A-2004180241 describes a coated cemented carbide cutting tool insert with large requirements on wear resistance and toughness behaviour of the cutting edge particularly suitable for general turning of stainless steels. In one embodiment, the substrate has a γ-phase depleted and binder phase enriched surface zone down to a depth of 5 to 50 μm. The composition of the substrate is 7.0 to 10.5 wt-% Co, 4.0 to 9.0 wt-% cubic carbides of elements from group IVb, Vb or VIb of the periodic table preferably Nb, Ta and/or Ti, a nitrogen content of 0.01 to 0.2 wt-% and balance tungsten carbide, WC. The coating contains an innermost layer system of up to three layers of TiCxNyOz (x+y+z≦1), a second multilayer system comprising 5 to 31 alternating layers of Al2O3 and TiCxNyOz (x+y+z≦1), preferably 11 to 15 alternating layers of κ-Al2O3 and TiN, and an outermost layer system comprising one or more layers of TiCxNy (x+y≦1) or three layers in sequence TiN—TiC—TiN or combinations thereof. Further, the outermost surfaces of the coated insert are mechanically post-treated so that the multilayer and partly the innermost layer system is exposed along the edge line.
In view of the state of the art there is a need for a cutting tool insert for machining steels at high speeds in general and stainless steels in particular. This refers especially to a cutting insert which exhibits an improved resistance against abrasive and adhesive wear, plastic deformation, chip hammering and chip jamming damages.